1. Field of the Invention
The present invention relates to a method of detecting a start of combustion (SOC) in a diesel engine using a pressure inside a combustion chamber (hereinafter, referred to as “in-cylinder pressure”), and more particularly to a method of estimating the start of combustion (SOC) in the diesel engine using a difference between an in-cylinder combustion pressure and a motoring pressure (i.e., a pressure inside a cylinder at a cycle where combustion does not occur) so as to effectively control the combustion in a combustion chamber.
2. Background of the Related Art
FIG. 1 is a graph illustrating a change in injection command, injection rate and rate of heat release (ROHR) in a combustion chamber over time in a diesel engine according to the prior art.
As shown in FIG. 1, a start of energizing (hereinafter, referred to as “SOE”) is obtained by defining a point of time when a command is issued to a solenoid of an injector or a piezo-actuator from an engine management system (EMS) on the basis of a crank angle.
Also, a start of injection (hereinafter, referred to as “SOI”) is obtained by defining a point of time when a nozzle of the injector is actually opened to inject fuel into a combustion chamber of an internal combustion engine on the basis of a crank angle, and the start of combustion (SOC) is obtained by defining a point of time when the quantity of heat corresponding to 1% of the total amount of heat released is released on the basis of a crank angle.
In this case, the time interval between the SOE and the SOI is defined as an injection delay, and the time interval between the SOI and the SOC is defined as an ignition delay.
In this regard, fuel injected is vaporized and mixed with air during the ignition delay time interval so as to be decomposed into a new chemical component that induces self-ignition.
Therefore, the SOC is determined by the relation of SOC, SOE, injection delay and ignition delay.
The SOC is generally known as having an effect on the overall performance of the engine.
That is, in case where the combustion early starts prior to a top dead center (TDC) position of a piston in the engine cylinder, the combustion pressure sharply increases to thereby prevent the piston from upwardly moving to the TDC position, which results in a deterioration of combustion efficiency and an increase of the engine operating sound generated.
In this case, the combustion must be completed before an exhaust valve is opened for the purpose of improvement of combustion efficiency and reduction of toxic exhaust emissions.
FIG. 2 is a graph illustrating the relationship between a variation of the SOC and the amount of NOx/HC emissions of a diesel engine.
As shown in FIG. 2, in case where the timing of the SOC is advanced, the maximum combustion temperature increases to increase the amount of NOx emissions into the atmosphere, whereas in case where the timing of the SOC is retarded, the combustion is not completed prior to the opening of the exhaust valve. As a result, a large quantity of unburned hydrocarbon (UHC) is produced.
In addition, since a mixing quality of the air-fuel mixture depends on the SOC, it affects the generation of soot as a byproduct of incomplete combustion.
As such, since the amount of HC and NOx emissions has a tendency of being inversely proportional to each other depending on the timing of the SOC, an optimal SOC value for all the operating conditions must be maintained in order to concurrently reduce the amount of HC and NOx emissions under all the operating conditions.
Almost all the mass-produced diesel engine fuel injection systems, which are controlled under a feed-forward SOC control scheme, are generally designed to perform a basic control strategy in which combustion efficiency and engine power increase in a full load condition and the amount of toxic exhaust emissions decreases in a part load condition.
However, since the existing diesel engine fuel injection systems do not measure an actual SOC, the SOC is controlled indirectly through the control of the SOE, but not controlled directly. Thus, although an appropriate SOE is determined upon the mass-production of the fuel injection systems, when an unexpected change occurs in the injection delay time and the ignition delay time, the timing of the SOC is not controlled at a desired point of time.
Since the injection delay time varies depending on a production tolerance of an injector, a rail pressure, etc., and the ignition delay time varies depending on the amount of diesel fuel used, the temperature of intake air introduced into each individual combustion chamber, compression ratio, etc., a look-up table is mainly used to compensate for the variations of the injection delay time and the ignition delay time and numerous experiments are required to write the look-up table.
Further, in such a feed-forward control, viscosity of fuel injected or design tolerance, aging of the engine and injector components, etc., cause a variation in the SOI or the SOC, resulting in making it impossible to further accurately control the SOC by using a fixed SOE look-up table.
Nevertheless, the range of permissible tolerances can be controlled strictly to alleviate the above problem upon the production of the injector, but a precise production process is required and hence the production efficiency decreases and the manufacturing cost increase. Moreover, the feed-forward SOC control cannot compensate for cylinder-by-cylinder and cycle-by-cycle variations in the injection delay time and the ignition delay time.
Accordingly, a variation in the both delay times must always be compensated for an accurate fuel injection system. To this end, a feedback SOC control and an accurate SOC measurement or estimation is needed.
In this regard, there have been proposed various methods of detecting the SOC in a diesel engine.
The SOI shown in the graph of FIG. 1 can be identified by measuring a lift displacement of an injector needle. However, this method entails a problem in that the manufacturing cost and complexity of the injection system increases to cause a deterioration of durability. Also, since such a method is aimed at detecting the SOI, but not detecting the SOC directly associated with the performance and exhaust emission quality of the actual engine.
Alternatively, in order to directly detect the SOC, there have been proposed and used an optical combustion timing sensor for detecting the SOC using the intensity of light of a flame burnt, an SOC sensor for detecting the SOC based on an ionization technique, i.e., a method of detecting the SOC using the strength of ionization current generated during the combustion, etc.
However, the direct detection method of the SOC using the optical combustion timing sensor and the SOC sensor based on the ionization technique embraces problems in that sampling performance and durability of the sensors are deteriorated due to contaminants such as soot generated in the diesel engine, thereby decreasing accuracy of the sensors in long-term use of the sensors. Furthermore, the both sensing methods also have a problem in that a sampling area is restricted around the tip of the sensors, so that all the areas within the combustion chamber of the diesel engine cannot be covered.
In the meantime, there is proposed a method of directly estimating the SOC through information on the pressure inside the combustion chamber or the cylinder, such as analysis of released heat as an alternative method of estimating the SOC.
But, since the heat release analysis based on a law of thermodynamics is very complicated mathematically as well as employs a very low reference value level (1%), it is susceptible to a noise. In order to compensate for this, the analysis of the heat release employs an average value of pressure signals generated during several tens of cycles, and hence has a slow response speed.
In order to improve the response speed estimated in such a method, there is proposed a method of defining the SOC as a point of time when 50% of mass of a mixture of fuel and air is burnt on the basis of a crank angle.
Such a method is robust to a noise so that a response speed is relatively high as compared to the SOC estimation method using the heat release analysis, but has a problem in that a calculation process is still complicated and the relationship is not consistent between the SOC and a crank angle corresponding to the combustion of 50% of mass of a mixture of fuel and air, so that there exists a difference between an actual SOC and an estimated SOC.
In addition, as another alternative approach of estimating the SOC, a research has been conducted on an SOC estimation method using the location of the center of gravity (CG) of a difference pressure (hereinafter, referred to as “DP”). Here, the DP is defined as a difference between the in-cylinder combustion pressure and the motoring pressure.
However, the above SOC estimation method using the coordinates of the center of gravity (CG) of the DP is used for the purpose of on-board diagnosis (OBD) but not control since a deviation of the estimated SOC is considerably large.
There is therefore a need for a method in which the SOC is practically estimated more accurately and simply and a response speed is sufficiently high enough to be applicable in real time.
However, the above-mentioned conventional techniques such as the measurement of the SOC using the optical combustion timing sensor or the ionization current SOC sensor, the estimation of the SOC using the heat release analysis encounter a problem in that sufficient accuracy and speed cannot be obtained for the purpose of the feedback control in the detection of the SOC.